This invention relates to electrical dust collecting apparatuses and more particularly to an improvement of such a dust collecting apparatus.
In electrical dust collecting, a high DC voltage is applied to a discharge electrode to cause corona discharge thereby to charge dust in a gas, the dust thus charged is separated from the gas by an electromotive force in an electric field, and the dust thus separated is collected by a dust collecting electrode.
In this type of electrical dust collecting, the dust collecting performance is most greatly affected by the electrical resistance of dust. If dust has an electrical resistance higher than 10.sup.11 .OMEGA.-cm, it is difficult for a conventional DC type dust collecting apparatus to collect the dust unless humidity control is carried out by spraying water or steam into the gas.
For instance, dust in a dry cement waste gas (essentially containing CaO, dust such as lead oxide, zinc oxide, ferrosilicone and ferrochrome), in various electric furnace waste gases, dust in a waste mud incinerator waste gas, and dust in a sintering furnace waste gas have an extremely high electrical resistance of the order of 10.sup.11 to 10.sup.14 .OMEGA.-cm in a certain temperature range (100 -200.degree. C. for instance) and in a certain humidity range (lower than 15% for instance).
The operation of the conventional DC type dust collecting apparatus to collect dust having such a high electrical resistance will be described. If negatively-charged dust is accumulated on its dust collecting electrode, the surface of the dust thus accumulated has a high negative potential. Therefore, negatively-charged dust on the discharge electrode is electrostatically repelled by the negative potential of the surface of the negatively-charged dust layer on the dust collecting electrode, thereby lowering its dust collecting performance. At the same time, a space charge effect takes place for the same reason, thus further lowering the dust collecting performance.
If the negative potential of the surface of the dust layer is increased to an extent that it ionizes the gas molecules in the dust layer, a so-called "back corona phenomenon" takes place and positive gas ions are therefore created on the surface of the dust layer. As a result, the charge of the negatively-charged dust on the discharge electrode is neutralized, and the dust collecting performance is increasingly lowered.
As is apparent from the above description, in collecting dust high in electrical resistance (higher than 10.sup.11 .OMEGA.-cm for instance) according to the DC charging system, as the surface potential of a dust layer is increased, dust charged with the same polarity is repelled thereby, thus lowering the dust collecting performance.
In order to overcome this difficulty, an AC charging system has been employed. However, in this case, dust is merely agglomerated by dust mutual collision due to the alternation in polarity of the line frequency, and therefore the dust collecting effect is not more than 60%. Thus, a high dust collecting effect cannot be expected.
In order to improve the dust collecting effect, a method has been thought of in which grid elements insulated from a dust collecting plate are provided in the space between the dust collecting plate and the discharge electrodes, and a voltage induced in the grid elements by AC polarity alternation is utilized for collecting dust. The grid elements are arranged in the form of a wire net. In either case, it has been ensured that even dust high in electrical resistance (higher than 10.sup.11 .OMEGA.-cm) can be sufficiently collected. However, this method is still disadvantageous in that as dust is deposited on the grid elements, the voltage induced in the grid elements is increased, and finally spark discharge occurs, as a result of which the dust collecting performance is extremely lowered.